In October 2016, the coalition-backed operation to recapture the city of Mosul was initiated. Notwithstanding its early territorial successes, it soon became clear that the campaign would take much longer than its predicted two months. The so-called Islamic State (IS) had been anticipating and preparing for it for quite literally years and, as such, advancing units were met with stiff, asymmetric resistance, an unprecedented defence that to a large extent relied on the widespread application of suicide tactics.

In the first seven days alone, no fewer than 58 IS fighters were reported to have killed themselves in suicide operations and, in the ten weeks that followed since, 179 more met similar fates.

While shocking, these numbers are not surprising. For years now, IS has been a suicide operation outlier, having consistently used human bombs far more regularly than any other organisation in history, jihadist or otherwise. In the summer, for example, 17 times as many of its fighters were reported to have died in suicide operations than its closest ideological rival in Syria, the al Qaeda-aligned group Jabhat Fath al Sham.

If previous months are anything to go by, as the Mosul offensive progresses in 2017, suicide tactics will continue to define IS’s defence, especially as its hold becomes more contested. With this in mind, I below offer some observations on the group’s three main suicide modi operandi.

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1. Vehicle-Borne IEDs

The majority of IS’s suicide fighters die in Vehicle-Borne IED (VBIED) operations, a term that refers to attacks perpetrated using one of nine types of vehicle: cars, lorries, tankers, Hummers, BMP infantry combat vehicles, tanks, armoured personnel carriers, bulldozers, and motorbikes. On occasion, IS also carries out ‘thana’iyya’ (joint) suicide attacks using two fighters – one who drives the VBIED while the other shoots from it.

The vehicle used varies depending on the context. For example, a tank-borne IED is much more useful in a battlefield setting than a scooter-borne bomb, which is itself better placed for smaller, more targeted terrorist attacks. In the majority of cases, IS’s vehicle of choice is the armoured four-wheel-drive. Given that most suicide operations are geared towards conventional battlefield goals, this is logical – such IEDs are relatively cheap to construct and can cover larger distances in a short amount of time, thus giving them an element of surprise. In Mosul, for example, they have proved to be ideal for launching against advancing ISF units from concealed garages. By way of contrast, because of their more cumbersome nature and the greater costs associated with their production, heavier VBIEDs (like armoured personnel carriers and bulldozers) have tended to be used more sparingly. While less agile, they can be both more powerful and more difficult to defend against and, as such, have played an integral role in the early stages of major IS offensive and defensive operations.

In the first week of the Mosul campaign, all of the defensive suicide attacks claimed by IS bar three used VBIEDs, a figure that is broadly consistent with trends that have emerged over the last few years.

2. Inghimas Operations

A derivation of the Arabic word ‘ghamasa’, to submerge, ‘inghimasi’ literally means ‘one who plunges’. While IS’s wholesale adoption of this tactic only appeared recently, the notion of inghimas is a famous one, owing to a substantial body of jihadist literature on it that dates back many hundreds of years. Ibn Taymiyya, for example, wrote a 79-page treatise on the subject – Qa’ida fi-l-inghimas fi-l-‘adu wa hal yubah? – and an entire chapter is devoted to it in Ibn al-Nahaas al-Dumyati’s work, Mashari’ al-ashwaq ila masari’ al-‘ushaq. In both instances, the authors argued that it is not only permissible but desirable for the mujahid to proactively risk their life when attacking a more numerous and better equipped enemy (provided, that is, that their intention is sound). These works, and others like them, are routinely pilfered by modern-day salafi-jihadist jurisprudents seeking to religiously justify the use of suicide bombers today, foremost among them, IS’s most important theological influence, Abu ‘Abdullah al-Muhajir.

In the specific context of IS, inghimas attackers are distinct from suicide bombers. The term refers to special operations involving fighters that willingly put themselves in harm’s way, maximising the risk of their death in order to cause as much damage as possible. In this sense, inghimas operations qualitatively differ from ‘traditional’ suicide attacks, because their success does not necessitate the perpetrators’ death, although it does make it highly likely.

In August 2015, an official IS video defined inghimas attacks as those in which:

one or more people plunge into an enemy position in which they are outnumbered, usually resulting in their death. Inghimas operations usually target fortified locations or urban building to kill important leaders. The target is chosen after careful surveillance and minute study of its surroundings and the amount of attention it receives from the enemy[.] Inghimas operations are considered to be a lethal weapon by which to make the enemy shudder[.] As such, just one inghimas fighter can make an entire army collapse.

For IS, there are at least three categories of inghimas. The first is the ‘battlefield inghimas’, in which its suicidal operatives swarm an enemy position, attacking it with light weapons and occasionally detonating suicide belts. In this iteration, which Scott Atran came across in the course of his fieldwork in Iraq, the inghimas attackers seek out death, but not by their own hand – as such, ‘martyrdom,’ while highly likely, is not a foregone conclusion. Indeed, reflecting this on a number of occasions in 2016, IS reported that one or more of its inghimas fighters safely returned back to their bases after an operation.

The second variant is the ‘psyop inghimas,’ which is geared, more than anything else, towards undermining enemy soldiers’ morale. As such, they tend to be launched as surprise attacks against camps and barracks. A notable pair of examples came in January 2016, when eight inghimas fighters launched a night-time raid on Camp Tariq near the Iraqi city of Fallujah. According to IS’s operation claim and subsequent article in al-Naba, the attack lasted for three hours and resulted in the deaths of 30 ISF soldiers. The day after, another such attack took place, this one targeting cadets housed at Camp Speicher. According, again, to the operation claim and subsequent al-Naba article, seven inghimas fighters infiltrated the camp in the early hours of the morning, using grenades and light weapons to kill as many people as possible before detonating their explosives-laden vests four hours later.

Neither of these assaults was geared towards capturing territory – rather, their primary value was propagandistic. In this sense, the October incursions in Kirkuk and Rutba could be considered to be ‘psyop inghimas’ attacks (though IS would probably argue otherwise).

The last form – the ‘terrorist inghimas’ – occurs in a civilian setting, wherein suicide fighters attack soft targets with light weapons before detonating suicide belts or vests (if they have them). The November 2015 Paris attacks were an example of this, as were the January 2016 assault against the Pakistani consulate in the Afghan city of Jalalabad, and July 2016’s attacks against Dhaka’s Holey Artisan Bakery and Kabul’s Deh Mazang square. Because they only featured suicide bombers, the March 2016 Brussels bombings should not be considered terrorist inghimas operations.

While there have only been a handful of inghimas attacks reported in the context of Mosul so far, this number is likely to balloon in the coming months. After all, the inghimas modus operandi makes great fodder for propaganda, offering a high impact, low cost way to buoy battlefield morale.

3. Human-Borne IEDs

The last manifestation of IS suicide tactics is also the most conventional. Comprising standalone events in which operatives use explosives-laden belts, bags or vests to target, in most cases, civilians, only one in ten IS suicide attacks take this form. Chiefly geared towards terrorist, not military, goals, human-borne bombs remain a cornerstone of the group’s directed propaganda of the deed efforts.

While they most commonly occur in Iraq – particularly, of late, in Baghdad – IS’s overseas affiliates have staged this form of attack in Africa and Europe too, hitting soft targets with a view to undermining the state and polarising the civilian population. As IS’s insurgent prospects dwindle, we must anticipate an acceleration – or at least attempted acceleration – in its adoption of HBIEDs because, similar to the terrorist inghimas, this tactic offers a psychological reserve upon which IS can call when the going gets tough.

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Currently, the vast majority of IS’s suicide operations are geared towards achieving military goals: some are offensive in nature, part of a broader tactical attempt to gain territory from adversaries or break through fortifications; others are perpetrated in pursuit of psychological, not territorial gains; yet more are defensive, an effort to thwart enemy advances and pre-empt counter-attacks. In a minority of cases, suicide tactics are used to target civilians in terrorist operations. As the group’s military prospects dwindle in Iraq and Syria, it is likely that this number will increase.

In any case, the extent to which Abu Bakr al-Baghdadi’s organisation resorts to suicide tactics challenges the conventional wisdom. The most prominent academic forays into the tactical and strategic thinking behind militarised suicide do not seem to fit the IS model – something borne of the fact that there is simply no unified explanation in this context. Indeed, suicide tactics are not just used to signal resolve, outbid rivals, spoil negotiations, undermine states or attract rewards. For IS, they are all of the above and more, an intrinsic, innovative part of its asymmetric warfare toolbox that cannot be explained by any one theoretical model. It is crucial that we recognise this, especially as the IS organisation hastens its tactical evolution in response to its flagging insurgent prospects.

Charlie Winter is a Senior Research Fellow at ICSR. Follow him at @charliewinter